1. Field of the Invention
The present invention relates to a steel sheet for a tension mask incorporated in color cathode ray tubes of televisions or computers, a making method thereof, and a tension mask using the same.
2. Description of Related Arts
Some color cathode ray tubes of televisions or computers incorporate a color selection electrode loaded with large tension like an aperture grill as a color selection mechanism, so-called a tension mask.
The tension mask is in general produced by subjecting a low carbon steel or an ultra low carbon Al-killed steel to hot rolling, cold rolling, continuous annealing, secondary cold rolling, and, as needed, to annealing for removing residual stress, followed by making apertures by photo-etching process, attaching to a frame by loading a tension of 200 to 400 N/mm2, for example, in one direction or two directions, and performing blackening treatment.
The blackening treatment is a heat treatment heating to temperatures of, e.g., 450 to 500xc2x0 C. so as to form an oxide film of magnetite on the steel surface, aiming at prevention of rusts on the tension mask or reduction of heat radiation, and at this time the tension of the mask sometimes lowers due to creep phenomenon of the steel sheet used to the tension mask. When the tension goes down, an aperture position of the mask may shift, the mask becomes easy to be resonated by speaker sound, and an electron beam does not land on a designated position, resulting in so-called xe2x80x9ccolor-deviationxe2x80x9d.
For preventing the creep phenomenon, JP-A-62-249339, (the term xe2x80x9cJP-Axe2x80x9d referred to herein signifies xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d), JP-A-5-311327, JP-A-5-311330, JP-A-5-311331, JP-A-5-311332, JP-A-6-73503, JP-A-8-27541, JP-A-9-296255, and JP-A-11-222628 disclose methods of adding elements such as Mn, Cr, Mo to steels for tension masks, or making much N solute in steel sheets for restraining climb motion of dislocation.
Further, recently accompanied with cathode ray tubes becoming large scaled, highly defined and flat, other than xe2x80x9ccolor-deviationxe2x80x9d caused by creep phenomenon of steel sheets, there has appeared a problem as xe2x80x9ccolor-deviationxe2x80x9d also caused by discrepancy in orbits of electron beams due to external magnetic field such as the earth magnetism.
For preventing xe2x80x9ccolor-deviationxe2x80x9d by the external magnetic field, it is effective to improve the magnetic shielding property of tension masks, and as methods therefore, JP-A-63-145744, JP-A-8-269569 and JP-A-9-256061 show methods of adding Si to steels for tension masks, JP-A-10-219396 shows Cu addition, and JP-A-10-219401 discloses Ni addition.
However, any investigations have not been made on the magnetic shielding property against external magnetic field in JP-A-62-249339, JP-A-5-311327, JP-A-5-311330, JP-A-5-311331, JP-A-5-311332, JP-A-6-73503, JP-A-8-27541, JP-A-9-296255, and JP-A-11-222628.
In particular, the methods described in JP-A-63-145744, JP-A-8-269569, JP-A-9-256061 and JP-A-10-219396 improve the magnetic shielding property, but because of containing Si and Cu, surface defects easily appear on the steel sheet at hot-rolling or recrystallization annealing, and therefore these methods are unwelcome in the application to tension masks requiring severe surface property.
The method of JP-A-10-219401 invites cost-up of steel sheets and deteriorates etching property due to the Ni addition.
The object of the present invention is to provide a steel sheet for a tension mask having excellent creep resistance and magnetic shielding property without deteriorating surface property or etching property, a method of making the same, and a tension mask using such a steel sheet.
The object of the present invention can be accomplished by a method of making a steel sheet for a tension mask, comprising the steps of: hot rolling a steel consisting essentially of, by weight %, C: less than 0.1%, Si: 0.05% or less, Mn: 0.4 to 2%, P: 0.03% or less, S: 0.03% or less, sol.Al: 0.01% or less, N: 0.010% or more, and the balance of Fe; cold rolling the hot rolled steel sheet; and annealing the cold rolled steel sheet, followed by a secondary cold rolling at a reduction rate of 35% or higher.